Add in-memory MongoDB mock; unit-test WordRepository; replace WordRepositoryMock

[Copilot]

MongoDbContextMock.Db threw NotSupportedException, making it impossible to unit-test WordRepository directly. Tests relied on WordRepositoryMock, a hand-written stub that bypassed all real query logic.

Approach: custom in-memory IMongoDatabase

Rather than Moq (can't evaluate FilterDefinition<T> — queries always return presets, hiding filter bugs) or EphemeralMongo/Mongo2Go (blocked by CI egress), this PR implements a lightweight in-memory MongoDB layer:

• InMemoryMongoCollection<T> — stores BsonDocuments; evaluates FilterDefinition<T> via a BSON filter evaluator supporting $and, $in, $exists, $elemMatch, and dot-notation. Auto-generates ObjectIds when Id = "", mirroring real MongoDB behavior.
• InMemoryMongoDatabase — lazily creates InMemoryMongoCollection<T> instances keyed by collection name.
• MongoDbContextMock — Db now returns InMemoryMongoDatabase; transaction mock provides a Moq-based non-null IClientSessionHandle required by MongoDB extension method null guards.

WordRepository unit tests

Backend.Tests/Repositories/WordRepositoryTests.cs — 37 tests exercising the real WordRepository against the in-memory backend: CRUD, frontier operations, IsInFrontier/AreInFrontier, domain counts, audio filters, and transaction rollback paths.

WordRepositoryTestHelper replaces WordRepositoryMock

WordRepositoryTestHelper wraps the real WordRepository (delegates all IWordRepository methods) and re-exposes the test-only surface previously on WordRepositoryMock:

// Test-only helpers, not on IWordRepository
Task<Word> Add(Word word)                  // insert into words only
Task<Word> AddFrontier(Word word)          // single-word frontier insert
Task DeleteAllWords(string projectId)
void SetGetFrontierDelay(Task<bool> delay) // for concurrency tests

All seven test suites (WordControllerTests, AudioControllerTests, MergeControllerTests, LiftControllerTests, WordServiceTests, StatisticsServiceTests, MergeServiceTests) updated to use WordRepositoryTestHelper.

Util.RandomWord() now produces valid ObjectId-format Ids (required by [BsonRepresentation(BsonType.ObjectId)]). MergeServiceTests hardcoded short IDs ("1", "ii", etc.) replaced with named 24-hex-char constants.

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This change is 

[imnasnainaec]

This was exploratory. Maintaining this fully mocked MongoDb looks like more work than keeping the necessary complexities of WordRepository synced up WordRepositoryMock.